1. Field of the Invention
The present invention relates to isolated polypeptides having acetyl xylan esterase activity and isolated polynucleotides encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides.
2. Description of the Related Art
Plant cell wall polysaccharides constitute 90% of the plant cell wall and can be divided into three groups: cellulose, hemicellulose, and pectin. Cellulose represents the major constituent of call wall polysaccharides. Hemicelluloses are the second most abundant constituent of plant cell walls. The major hemicellulose polymer is xylan. The structure of xylans found in cell walls of plants can differ significantly depending on their origin, but they always contain a beta-1,4-linked D-xylose backbone. The beta-1,4-linked D-xylose backbone can be substituted by various side groups, such as L-aribinose, D-galactose, acetyl, feruloyl, p-coumaroyl, and glucuronic acid residues.
The biodegradation of the xylan backbone depends on two classes of enzymes: endoxylanases and beta-xylosidases. Endoxylanases (EC 3.2.1.8) cleave the xylan backbone into smaller oligosaccharides, which can be further degraded to xylose by beta-xylosidases (EC 3.2.1.37). Other enzymes involved in the degradation of xylan include, for example, acetyl xylan esterase, arabinase, alpha-glucuronidase, ferulic acid esterase, and p-coumaric acid esterase.
Acetyl xylan esterase (EC 3.1.1.6) removes the O-acetyl groups from positions 2 and/or 3 on the beta-D-xylopyranosyl residues of acetyl xylan. Acetyl xylan plays an important role in the hydrolysis of xylan because the acetyl side groups can interfere sterically with the approach of enzymes that cleave the backbone. Removal of the acetyl side groups facilitates the action of endoxylanases. A classification system for carbohydrate esterases, based on sequence similarity, has led to the definition of 13 families, seven of which contain acetyl xylan esterases (Henrissat B., 1991, Biochem. J. 280: 309-316, and Henrissat and Bairoch, 1996, Biochem. J. 316: 695-696).
The present invention relates to an acetyl xylan esterase from Aspergillus aculeatus and polynucleotides encoding the polypeptide.
An acetyl xylan esterase from Aspergillus aculeatus is known from WO 1995/002689 (GENESEQP:AAR63066); however, this enzyme has no significant homology to the acetyl xylan esterase of the present invention.
An acetyl xylan esterase from Aspergillus niger is known from EP507369 (GENESEQP:AAR25291). This enzyme is 80% identical to the acetyl xylan esterase of the present invention.